The long-term objective of Project 4 is to understand the biological function of an integrin-regulated cytoplasmic tyrosine kinase, c-Abl. The c-Abl tyrosine kinase is important to mammalian development, because Abl-knockout mice exhibit a variety of developmental defects leading to embryonic or neonatal lethality. The activated Bcr-Abl tyrosine kinase of chronic myelogenous leukemia can promote integrin-independent cell survival and proliferation. Through collaboration within this Program, it has been shown that Bcr-Abl can stimulate the membrane recruitment of activated Cdc42 in the absence of integrin signals. We have also found that c-Abl can modulate the formulation of lamellipodia and filopodia during cell spreading on fibronectin. The c-Abl tyrosine can modulate the formulation of lamellipodia and filopodia during cell spreading on fibronectin. The c-Abl tyrosine kinase possesses three nuclear localization signals. During the current funding period, we have discovered a nuclear export signal in c-Abl and shown that cell adhesion can stimulate the nuclear exit of c-Abl. Stimulation of Abl nuclear export by integrin signals ay contribute to cell survival. This is because the nuclear c-Abl tyrosine kinase, when activated, can induce apoptosis. Interestingly, the oncogenic Bcr-Abl tyrosine kinase is retained in the cytoplasm of transformed cells. We have been able to trap Bcr-Abl in the nucleus and shown that the nuclear Bcr-Abl can also activate apoptosis, leading to the irreversible eradication of leukemia cells. The c-Abl protein contains an F-actin binding domain. We have found that F-actin is an allosteric inhibitor of the Abl kinase. Interestingly, c-Abl mutants defective in F-actin binding retain kinase activity in detached cells and can stimulate the formation of membrane protusions in the absence of integrin signals. To pursue these observations, we propose to investigate how integrins activate the c-Abl kinase activity in the context of the F- actin mediated inhibition. We will also investigate how integrins activate the nuclear export of c-Abl. In collaboration with Project 3, we will investigate the regulation of Rac and Cdc42 by c-Abl and Bcr-Abl and determine the effect of Abl tyrosine kinase on cell migration. We propose to develop tools that can be used in collaboration with Projects 1 and 2 to examine the role of c Abl kinase in the integrin-dependent outside-in and inside-out signaling pathways. The proposed research will rely on the protein purification and the imaging cores of this Program. Through collaboration among the projects and cores, this Program is uniquely qualified to investigate the functions of Abl tyrosine kinase in the normal development of blood cells and in the pathological development of leukemia.